When wind turbines are erected it is normal and feasible to use a large mobile crane, although such use is costly and may involve problems in getting the crane to the erection site. Erection sites are often remote and difficult to access, but since a tower, and a nacelle, a hub and blades etc. must all be lifted up for installation a mobile crane with a lifting yoke is normally preferred for eased and quick installation. The benefit is even larger if many turbines are to be erected for a wind park.
In case a single turbine blade, or a limited number of blades, have to be dismounted and refitted, e.g. in a rare situation for repair or replacement, the cost of sending a mobile crane is relatively very high as the cost is at a similar scale whether one blade has to be taken up and down or a whole turbine is to erected. Hence, it is preferred to perform such limited operations without a crane, especially seen in the light of modern wind turbines have a tower height extending beyond 100 meters, which requires very large cranes.
A number of solutions have been proposed for craneless dismounting and mounting blades. One proposal is shown in U.S. patent application 2010/0139062, where two pulleys are mounted externally on the hub of the wind turbine. A blade is lifted in a harness by the use of wires extending up from the ground via the pulleys and down to the harness to lift the blade. Such a solution requires that personnel must leave the nacelle or hub in order to fasten the pulleys to the hub and to arrange the wire extending from the ground over the pulleys.
In EP 2159419 is it suggested to use a ground based winch and run a lifting wire from the winch to the nacelle and via a number of pulleys over a main shaft into a position outside or inside the hub. In both embodiments a hoisting jig is used which is connected to the blade via so-called T-nuts, which are metal inserts placed in through-going holes in the blade wall and arranged for receiving bolts for attaching the blade to a wind turbine hub. The hoisting jig for exterior mounting would for dismounting of blades require personnel to leave the hub for engaging the jig. The embodiment shown for internal mounting requires three eye plates for engaging the blade metal inserts in the through-going holes. This requires a tripod-like frame structure of a considerable size, which may be difficult to get into the hub without removing a blade first. Moreover, the required through-going holes having the metal inserts, which the hoisting jig is attached to, may seriously affect the strength of the blade as the maximum bending moment, both edgewise and flapwise—i.e. from wind pressure and gravity, is present at the blade root portion.
As turbine blades are normally preferred to be installed using a crane with a lifting yoke, the blades will mostly be without any specific structure adapted for receiving a crane hook or other similar rigging equipment. This is first of all because it is not expected that a blade has to be dismounted and because such a structure would a cost and weight to the blade.
There are a number of examples where blades have been fitted with holes, as in EP 2159419, for lifting or handling the blades. However, unless the holes are filled with an insert as in EP 2159419, other devices have to be used to close said holes to prevent water from penetrating or negatively influence the aerodynamics of the blades.                One object of the invention is to enable dismounting and mounting one or more blades from a hub attached to a nacelle placed on a tower, and performing this operation without the use of a separate crane, e.g. a mobile crane.        Another object of the invention is to avoid personnel having to leave the hub or nacelle to attach or detach e.g. lifting straps.        A further object of the invention is to avoid having to place various devices such as pulleys on the outside of the hub.        A yet further object is to avoid having to equip the blade with lifting means such as straps to be wrapped around the blade, or to equip the blade with e.g. holes for engaging lifting means to the blade, or to avoid providing the blade with an internal engagement structure to which lifting means may be engaged.        Other aspects, objectives and advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.        